It'll be difficult to put my top five into the traditional 5-1 format but I'll do my best. I may not rank them numerically:
Legend of Zelda: Link to the Past: I had some trouble here because I like Ocarina of Time just as much as Link to the Past. I chose LttP however, because of the nostalgia and childhood memories associated with it. OoT has similar nostalgia but I was younger when I had LttP. I recently replayed LttP and beat it in about 6 hours. I reflected on how it took me days to beat certain dungeons! OoT also lost out because it's the same exact formula as LttP. You start in a bright, happy world and must find 3 maguffins. You get the Master Sword and are sent into an alternate world wherein you must collect 7 maguffins before you can fight Gannon.
Pokemon: I really can't pick a particular one since each sequel builds up from the last one. Red, Silver, Sapphire, Diamond, Soul Silver, Heart Gold, White and, Black. I've been playing Pokemon since I was in Elementary school and don't think I'll ever give it up, despite it's somewhat lackluster plot and the fact that it's trying to give me OCD.
Duke Nukem 3D/ Duke Nukem 64: Everyone of a certain age fell in love with Goldeneye for the N64. I loved that game too but when it came to first person shooters, Duke beat out Bond in a BIG way. I would put on Goldeneye if I wanted to breeze through a level with invincibility, unlimited ammo and, a pair of shotguns. I played Duke Nukem for 3 or more hours at a time for the bot matches. Duke Nukem 64 was the first game I ever played until 4am as a tender, young, 4th grader.
Star Wars: Knights of the Old Republic: This game came out in time to whet my appetite for more Star Wars in the wake of the release of the New Trilogy. Right after Star Wars: Attack of the Clones, Bioware released the first of two Knights of the Old Republic games. Even then I knew and would talk about how much better the plot for those 2 games are than the collective plots of the new trilogy. This single game got me into games by Bioware, books by Drew Karpyshyn and, the Old Republic mythology in general. I still have yet, however, to build a Jedi or Sith gunslinger.
Saint's Row 2: I loved Grand Theft Auto III, Vice City and San Andreas and as beautiful as Grand Theft Auto IV was, I was underwhelmed by it and it's lack of silliness. Compared to the 3 PS2 entries GTAIV was practically a conservative game. In-comes Saint's Row 2, a game dripping in cartoon silliness. My current character is a well-dressed man with an angry woman's voice (female 2) and, enough facial piercings to make any alarm go off. These games really touch a primitive part of the brain; the one that demands you to fling poo at yachts and, throw old people off of roof-top gardens...while naked...then flinging yourself off of the same roof...while on fire. The best thing about this game is that unlike other current gen games, you unlock cheats as you play (unlimited ammo, no damage from falling, reduced fire damage, reduced bullet damage, 3X health regeneration, etc) all of those and more which can be active and you still get to unlock achievements.
If you forced me to rank these games numerically, I think I'd go:
1) Pokemon
2) Saint's Row
3) Knights of the Old Republic
4) Zelda
5) Duke
